Foundations and Applications

Friedrich August Kekulé

German chemist Friedrich Kekulé, known for his work and
theories in molecular structure, such as the tetravalent structure of
carbon.

GERMAN CHEMIST
1829–1896

Friedrich August Kekulé was born on September 7, 1829, in
Darmstadt, Hesse (later part of Germany). He showed an early aptitude for
both languages and drawing and wanted to be an architect. He began his
architecture studies at the University of Geissen in 1847, but after
attending the lectures of the famous chemist Justus von Liebig he switched
to chemistry. Kekulé had great interest in the theoretical aspects
of chemistry, and less in the more practical applications that so
interested von Liebig. On von Liebig's advice Kekulé went to
Paris in 1851 to further his chemical studies.

Paris during the 1850s was an ideal place for a young scientist, as there
was a great deal of interest in that city in theoretical chemistry,
particularly in the structure of molecules. Preexisting ideas (such as the
dualism of Swedish chemist Jöns J. Berzelius) stressed that
molecules formed because of the inherent electrical charges that
individual elements possessed (which were sometimes opposing and therefore
attractive). Organic molecules were not in keeping with the dualism
concept, but some scientists proposed that they could be derived from a
number of simple inorganic molecules.

Kekulé returned to Germany in 1852, obtained his doctoral degree at
Geissen in that year, and then spent a year working in Switzerland. This
was followed by two years in London (1853–1855), where he met the
chemist Alexander Williamson. Williamson had extended Charles
Gerhardt's "type theories" to explain how ethers
could be derived from the water type. Kekulé, with
Williamson's encouragement, extended type theory further and
introduced a new type—the methane or marsh gas type. This led to
the development of the tetravalent model of carbon and the understanding
that carbon forms rings and chains.

Kekulé's principal insight was to realize that type theory
did not take into account the specific combining power (or valences) of
specific atoms. In 1857 Kekulé suggested that carbon was
tetravalent, his suggestion based on the specific chemistries of the
compounds that carbon formed with elements such as hydrogen (CH
4
) and chlorine (CCl
4
). Kekulé extended his ideas in the following year by suggesting
that two carbon atoms bonded together in the formation of hydrocarbons
such as ethane (C
2
H
6
). Similarly, additional carbon and hydrogen units could be added,
extending the carbon
atom chain and forming an ordered series. A similar structural theory was
developed independently at around the same time by the Scottish chemist
Archibald Scott Couper, working in the laboratory of Adolphe Wurtz in
Paris. Publication of Couper's paper was delayed by Wurtz (until
Kekulé's had appeared) and the structural theory of organic
chemistry is really a culmination of the efforts of Kekulé and
Couper.

Kekulé was offered the position of professor of chemistry at the
University of Ghent in Belgium in 1858. There his linguistic abilities
stood him in good stead, as he had to lecture in French. In 1867
Kekulé was called to the University of Bonn and remained there
until his death, on July 13, 1896.

In 1859 Kekulé started to use graphical representations of organic
molecules, in part to emphasize the tetravalent nature of carbon atoms and
their ability to form chains. He then turned his attention to the
structure of benzene (C
6
H
6
), a compound with unusual properties that could not be explained by any
theories of the day.

Kekulé proposed in 1865 that benzene had a structure in which six
carbon atoms formed a ring, with alternating single and double bonds.
However, the chemistry of benzene was not always consistent with this
structural formula. (All of the carbon atoms in a benzene molecule were
equal and equivalent in terms of the reactions of benzene.) To overcome
this problem, Kekulé suggested in 1872 that there were two forms of
benzene, in dynamic
equilibrium
. Kekulé's dynamical theory proved to be only partially
correct. In 1933 Linus Pauling used quantum mechanics to explain more
fully the nature of benzene.

On March 11, 1890, on the occasion of the twenty-fifth anniversary of his
announcement of his benzene theory, Kekulé gave a speech in Berlin
in which he revealed that both his structural theories and the structure
of benzene were revealed to him in dreams. Scholars have tended to dismiss
his account of his own creative processes and have placed more stock in
the early training that Kekulé received in architecture as the key
to his inspiration.

JOSEF LOSCHMIDT (1821–1895)

Before Kekulé dreamed of his structure for benzene, Josef
Loschmidt proposed the ring structure of the molecule. Unfortunately,
Loschmidt did not publish his theory in a widely read scientific
journal. As a result, the credit for this revolutionary theory is hotly
debated today.